Anaerobic digestion of organic matter may be used for different purposes and with many different types of organic materials. For example anaerobic digestion may be used for decreasing the volume of a waste material, such as a food production waste sludge or a domestic waste-water. Another example is the use of anaerobic digestion for to produce biogas using suitable crops. A general desire with all these processes is to obtain a high amount of biogas and a high degree of digestion, i.e. a low amount of organic material in the digested sludge.
EP 0 970 922 A2 describes a process for anaerobic digestion of organic waste, such as kitchen refuse. It is noted that sometimes the amount of trace metals available is not sufficient for the micro organisms involved in digestion and methane formation. EP 0 970 922 A2 proposes to add complexing agents and/or to add trace metals to arrive at a concentration of Co, Ni, Fe, Mn of about 1 ppm.
A problem with the process of EP 0 970 922 A2 is that it does not provide the optimum conditions for anaerobic digestion with different materials and operating conditions. Therefore the process of EP 0 970 922 A2 may sometimes provide too high consumption of trace metals still not providing for efficient digestion.